Polypropylene materials, formed by Ziegler-Natta or metallocene catalysts, are among the most versatile and commonly used thermoplastics in the world today. Polypropylene materials are useful in creating a great variety of finished goods including cast and blown films, injection molded parts, blow molded articles, thermoformed sheets, and fibers which may be subsequently spun or woven to create carpet and other finished goods. Although both polyethylene and polypropylenes are types of polyolefins, polypropylenes tend to be stiffer and exhibit higher yield stresses and melting points in comparison with polyethylenes but are also more prone to fracture, especially at low temperatures. This primarily results from higher glass transition temperatures, and may be addressed by producing a toughened blend using rubber or other polymeric impact modifiers to improve low temperature impact resistance at some sacrifice in modulus.
As noted earlier, nearly all commercial grade polypropylenes are produced using either Ziegler-Natta or metallocene catalyst mechanisms. These catalysts allow a certain degree of control in regard to the polypropylene's tacticity or arrangement of methyl groups extending from the carbon chain backbone of the finished polymer. A polypropylene molecule having a random arrangement of these pendant groups would be known as atactic. Whereas a polypropylene chain which always located the pendant group on the same side of the chain or in the same orientation would be known as isotactic, and one in which the pendant group alternated from one side of the chain to the other in a repeating pattern would be referred to as syndiotactic.
Traditionally, commercial polypropylenes have been isotactic as these tend to exhibit greater strength and stiffness in the finished product. However, relatively recent innovations in catalyst chemistry have enabled relatively large scale operations for the production of syndiotactic polypropylene. Although not as strong or as stiff as isotactic polypropylenes, syndiotactic polypropylenes offer a unique set of properties including greater flexibility, higher resistance to impact, and superior optical clarity.
There are a number of unique applications which are ideally suited to strong, flexible, and substantially clear polyolefins. By way of example only, plasticized polyvinyl chloride (PVC) has traditionally been used either alone or with other polymer components to form a number of medical articles including bandages, surgical dressings, and intravenous (IV) solution bags. Plasticized PVC films possess many desirable properties including easy stretch, high degree of recovery, low fatigue and minimal permanent set. However, plasticized PVC film has become less desirable because of known or suspected carcinogens associated with both the PVC monomer and the various plasticizers used in its production. Clearly, in medical articles, food storage containers, and other applications where polymers are either in direct contact with blood or other bodily fluids or in contact with food or other items which are to be ingested or taken into the body, it would be desirable to replace materials like plasticized PVC film with various polyolefins, particularly those with very low extractable contents.
Although syndiotactic polypropylene offers superior strength and optical clarity in comparison with less expensive polyolefins, namely polyethylene, sPP homopolymer is typically too stiff to be used in applications where softness or drapeability are critical factors. Additionally, because syndiotactic polypropylene does not crystallize as rapidly as isotactic polypropylene, it is somewhat less processable because molded articles require longer periods of time being held in injection molds or the like to retain their shape and ensure proper dimensional stability. Accordingly, there is a need for syndiotactic polypropylene blends which offer reduced stiffness, improved optical clarity, reduced injection molding cycle times and improved toughness.